He’s been hoping to leave the US Senate for bigger things for more than a decade. And now, his opportunity has finally arrived — but at the very time when his influence in the upper chamber and his importance to Massachusetts are at their zenith.

According to the Washington Post, President Obama may give Kerry a top job, but not secretary of state, the position the senator covets. The White House reportedly wants to put UN Ambassador Susan Rice in that post and instead is considering tapping Kerry to be the next secretary of defense.

For Kerry, a man given to agonizing over big decisions, this could be a tough call.

Still, the senator’s best course is clear. Given the power and prominence he now enjoys, secretary of state is the only post worth surrendering his Senate seat for. Being the nation’s top diplomat would play to his strengths; trying to manage the nation’s defense bureaucracy would not. Absent Obama offering him Foggy Bottom, Kerry should stay right where he is.

For Kerry, staying put will require growing comfortable with the notion that he is where he should be, after years of wanting to move on. Always easy to caricature, he earned an early reputation as an ambitious, headline-hungry senator chafing for a chance to move up.

Kerry thought hard about running for president in 2000, but ultimately decided against taking on a sitting vice president. When Al Gore secured the nomination, Kerry hoped to be his VP nominee, and was deflated when Gore passed him over for Joe Lieberman. In 2004, his chance to run nationally finally came, only to end in a tough loss to incumbent George W. Bush. His prospects for a second presidential campaign ended in 2006, unfairly, with a flubbed joke.

After Barack Obama won the presidency in 2008, Kerry hoped that the man he had helped introduce to the nation by giving him the keynote spot at the 2004 convention would name him secretary of state. Instead, in a grand gesture of Democratic Party reconciliation, that job went to Hillary Clinton.

Now, with Clinton ready to leave, the Washington Post reports that Obama is ready “to dig in his heels” to secure that post for Rice. Kerry could get defense, which would be a very substantial consolation prize.

But the Pentagon job isn’t worth giving up the role Kerry now has in the Senate.

Long in the Senate shadow of Ted Kennedy, a legendary lawmaker but also a competitive colleague who could be grudging about sharing the limelight, Kerry has truly come into his own. Since Kennedy’s death, he has filled the role of the state’s senior senator with an energy and attention to detail that has been a pleasant surprise to those who remember how easily distracted a younger Kerry had been. Once only average, his staff, both in Washington and Massachusetts, is now considered top-notch.

On the national stage, Kerry has won plaudits for his performance as chairman of the Senate Foreign Relations Committee, where he has managed to keep the hyper-partisanship that infects so much of Washington from hindering the panel’s work. That same ability was evident in the way Kerry helped win Senate ratification for the New START Treaty. He’s also performed skillfully on the ad hoc foreign policy missions he has undertaken for the Obama administration.

But that’s hardly his only role. Kerry, who is about to become the fourth-most senior Democrat in the Senate, is also a key player on both the Senate Finance and Commerce committees. In the former post, he helped craft Obama’s landmark health care bill.

And though last year’s super committee failed to hammer out a bipartisan deficit-reduction agreement, Kerry’s work there has positioned him to play a strong role on that issue, which will occupy Congress’s attention in the months and, probably, years to come. His past work has also prepared him to play a vital role on global warming.

The nation really doesn’t remember secretaries of defense, except perhaps for their failures. But we do remember legendary senators. And if he stays in the Senate, Kerry has an opportunity to join those ranks.